Lenticular sheets are used to give images an appearance of depth. More specifically, a lenticular sheet comprises a transparent upper layer A having narrow, parallel lenticulas (semi-cylindrical lenses) B on an outer surface, and an image-containing substrate layer C which projects images through the lenticulas. (See FIG. 1A). The two layers of a lenticular sheet provide an image such that different portions of the image are selectively visible as a function of the angle from which the lenticular sheet is viewed. If the image is a composite picture made by bringing together into a single composition a number of different parts of a scene photographed from different angles, and the lenticulas are vertically oriented, each eye of a viewer will see different elements and the viewer will interpret the net result as a three dimensional (3-D) image. The viewer may also move his head with respect to the lenticular sheet thereby observing other views with each eye and enhancing the sense of depth.
Another method for showing 3-D images is the use of a blocking line screen positioned at a specific distance from the composite picture. This process, known as a parallax process, causes blocking of all images except one specific image. This allows the eyes to view different images as three-dimensional (3-D) images, when the blocking line screen is oriented vertically.
When the lenticulas or the blocking line screen is oriented horizontally, each eye receives the same image. In this case, the multiple images give illusion of motion when the composite image is rotated about a line parallel to the viewer's eyes. Thus, a simulation of motion is achieved by the process of tipping the lenticular sheet or the blocking line screen, or by movement of the viewer's head to a different angle with respect to the lenticular sheet.
Whether the lenticulas or the blocking line screen is oriented vertically or horizontally, each of the viewed images is generated by lines of images (also referred to as image lines) which have been interlaced at the spatial frequency of the lenticulas or the blocking line screen. Interlacing lines of each image with other images is referred to as interdigitation. A full set of such interdigitated image lines forms a lenticular image. Interdigitation can be better understood by using an example of four images used to form a composite image with a material having three lenticulas. In this example, line 1 from each of the four images is in registration with the first lenticule; line 2 from each of the four images is in registration with the second lenticule; etc. Each lenticule is associated with a plurality of image lines D or an image line set (See FIG. 1), and the viewer sees only one image line of each set with each eye for each lenticule. It is imperative that the image line sets be registered accurately with respect to the lenticulas, so that the proper picture is formed when the assembly is viewed. One method of conventional recording of the interdigitated image lines requires recording of the interdigitated image lines on a recording material contained on the substrate layer C and then attaching the substrate layer C to the upper layer A, with the recorded image lines D in precise alignment to the lenticulas B to yield the desired image structure. The precise alignment of the specific lenticulas with the desired image line set during the attachment of the recording material to the lenticular overlay is difficult to achieve. This results in a degraded image quality.
Conventional recording of lenticular images has been accomplished with a stereoscopic image recording apparatus that uses optical exposure. A light source, such as a halogen lamp, is projected through an original image, via a projection lens, and focused on the substrate layer of the lenticular sheet. The lenticular images are exposed on a recording material as interdigitated image lines. Japanese (Kokoku) Patent Applications Nos. 5473/1967, 6488/1973, 607/1974, and 33847/1978 disclose recording apparatus in which two original images are projected for printing on a lenticular recording material. Recording lenticular images in this fashion (i) requires complex projection lens systems, which are expensive, and (ii) does not work well with thermal dye transfer approaches because it requires more power than what is produced by a halogen lamp or a similar light source.
In contrast, image recording by scanning (linear) exposure requires comparatively simple optics, yet has great flexibility in adapting to various image processing operations, and to alterations in the dimension of the lenticulas. To take advantage of these features, various apparatus and methods have been proposed for recording an image by scanning exposure. For example, Japanese (Kokoku) Patent Application No. 3781/1984 teaches a stereoscopic image recording system in which a plurality of original images is taken with a TV camera, processed and stored in frame memories from which the stored image signals are retrieved sequentially as image lines in accordance with the pitch of lenticulas used. After the image lines are recorded on a substrate layer by scanning exposure, the upper layer of the lenticular sheet is bonded to the substrate layer containing the image lines. Another image recording system uses polygon scanners, described in U.S. Pat. No. 5,349,419, for exposure of stereoscopic images directly on photosensitive back surface of a lenticular sheet.
It is desirable to write interdigitated images directly on a back surface of lenticular sheet using thermal dye transfer. This would eliminate the need for careful alignment of specific pre-printed image lines of the substrate layer with the specific lenticulas of the upper layer of the lenticular sheet. The use of thermal dye transfer to write such interdigitated images requires, however, large amounts of energy. Such energy can be provided by high power lasers.
Furthermore, high quality lenticular images require that a large number of images be placed behind a fine pitched lenticular sheet. For example, in order to produce 25 images with a lenticular sheet of 100 lenticulas per inch, one needs to produce 2500 lines per inch of continuous tone spots. This requires a pixel size of approximately 10 microns or less. To expose such a small pixel, the beam size has to be of approximately the same size as the pixel size. A single mode laser can easily provide such a small beam size. Therefore, a single mode laser may be used to write interdigitated images. Unfortunately, high power, inexpensive single mode diode lasers are not available.
The cross-referenced patent application entitled FLYING SPOT LASER PRINTER APPARATUS AND A METHOD OF PRINTING SUITABLE FOR PRINTING LENTICULAR IMAGES, by David Kessler et al, filed concurrently herewith discloses that this problem can be solved by utilizing a multimode laser to write interdigitated images. Such lasers provide the necessary high power and are relatively inexpensive. Unfortunately, a multimode laser beam provided by such a laser has a large cross section at a scanning mirror in order to provide the required small spot at the recording material. Therefore, the scanning mirror requires a very large mirror aperture. Hologon and polygon scanners are frequently used in flying spot printers. If such large mirror apertures can be avoided, hologon and polygon scanners could be made less expensive and bulky, and could be more easily maintained in a vibration free environment.
Galvanometer mirrors (galvo-mirrors) can provide a large mirror aperture and still be physically light and inexpensive. Resonant galvo-mirrors can provide speed and aperture but have a sinusoidal variation in writing pixel time creating very complex signal timing and dwell time problems. Non-resonant galvo-mirrors do not have this problem. However, non-resonant galvo-mirrors are relatively slow. Compensation for the slow speed of the non-resonant galvo-mirrors by printing with multiple laser beams provided by multiple lasers is also difficult, for the reasons described below.
Unfortunately, humidity and temperature variations cause the spacing between the lenticulas to change slightly, changing the pitch of the lenticular sheet. When this happens, the pitch of image lines needs to be adjusted so that correct image line sets correspond to the proper lenticulas. When one writes with a single multimode laser, one can easily change the spacings between each of the image lines by changing the translation speed (in a cross-scan direction) of the recording material. However, with the use of a galvo-mirror the printing speed becomes an issue. Galvo-mirrors are limited to about 200 Hz scanning frequency and thus become the limiting factor for printing speed.
To increase the printing speed in a system using a (non-resonant) galvo-mirror, it would be advantageous to simultaneously use more than one multimode laser. However, changing the spacing between the lasers so as to compensate for humidity and temperature variations is difficult and expensive. When the spacing of the image lines is maintained constant, and the width of the lenticulas changes, the registration of image lines and the lenticule is no longer correct (see FIG. 1B). This results in either the wrong viewing distance, or in scrambled images (i.e., a viewer will simultaneously see different portions of different images).